The present invention is a piezoelectric pressure sensor comprising a material having both a piezoelectric and pyroelectric output signal. The sensor further comprises electrical contacts having a geometry for enhancing the piezoelectric output signal of the sensor and for substantially eliminating the pyroelectric output signal. Although the sensor is disclosed as a zinc oxide (ZnO) on silicon diaphragm structure, the invention is not limited to such a structure.
Silicon diaphragms have been used in a variety of pressure sensor devices such as capacitive pressure sensors and piezoresistive pressure sensors; see, for example, C. S. Sander, J. W. Knutti, J. D. Meindl, IEEE Trans. Ed. 27, No. May 5, 1980. The use of silicon as part of a sensing element is especially attractive since it allows the silicon diaphragm and the electronic part of the device to be integrated on the same silicon substrate. Such a structure increases the accuracy and the stability of the device and lowers its cost. Further, etching techniques enable the making of thin diaphragms which increase the pressure sensitivity of such devices.
ZnO is a piezoelectric material which can be sputtered on a variety of substrates including silicon and metals, and ZnO can be used as the active element of such a pressure sensor; see, for example, P. L. Chen, R. S. Muller, R. M. White, R. Jolly, IEEE 1980 Ultrasonic Symposium.
When a silicon substrate on such a device is flexed, a stress pattern appears in the ZnO layer and induces a piezoelectric polarization parallel to the 3 axis of the ZnO lattice, which is perpendicular to the surface of the layer. A voltage output can be then collected across a capacitor formed by two electrodes deposited at the top and the bottom surfaces of the ZnO layer. Such a capacitive arrangement, however, typically presents the disadvantage of having to connect the top electrode to the electronics on the silicon substrate, thus involving a step coverage problem, especially in the case of thick ZnO layers on the order of 10 microns in thickness.
The other problem involved in using ZnO as the active element arises from the pyroelectric properties of ZnO, which produces a voltage output due to temperature variations.
The present sensor comprises an electrode configuration which not only solves the two problems mentioned above but also enhances the piezoelectric output signal from the sensor. Thus, the electrode configuration of the present sensor has three technical advantages over prior art piezoelectric pressure sensors. The electrode configuration of the present sensor eliminates the need to connect the top capacitative electrode to the electronics on the semiconductor substrate, thus eliminating a step coverage problem. In addition, the electrode configuration of the present sensor substantially eliminates the pyroelectric voltage output due to temperature variations. Further, the electrode configuration of the present sensor enhances the piezoelectric output signal generated by the piezoelectric sensing material in the sensor.